Lead-zinc battery

ABSTRACT

A rechargeable battery is provided such that the positive electrode comprises lead dioxide, the negative electrode zinc, and the electrolyte is alkaline. Upon discharge, the lead dioxide is reduced to lead oxide, the zinc is oxidized to zinc oxide, and the electrolyte remains unchanged. The reactions are reversed when the battery is charged.

RELATED APPLICATION

This application is a continuation of U.S. application Ser. No.10/756,015 filed under attorney docket no. STF-122-A on Jan. 13, 2004,currently pending.

FIELD OF THE INVENTION

The present invention relates to a novel type of storage battery whichis distinguished by its unique electrochemistry. The positive electrodecomprises lead dioxide and the negative electrode zinc. The electrolyteconsists of an alkaline aqueous solution of an alkali metal hydroxide ortetramethyl ammonium hydroxide to which various buffers, includingcarbonates, borates, silicates, and phosphates, may be added. Upondischarge the lead dioxide is reduced to lead oxide and the zinc isoxidized to zinc oxide.

BACKGROUND OF THE INVENTION

The most common storage battery, found in almost every vehicle, is thelead-acid battery. This battery comprises a lead dioxide positiveelectrode, a lead metal negative electrode, and sulfuric acid for theelectrolyte. Its chief advantage is low cost. Nevertheless, it has alimited energy density and the electrolyte is extremely corrosive.Furthermore, sufficient acid is required to react with the electrodesduring discharge. Maintenance-free types avoid the loss of evolvedgases, as disclosed in U.S. Pat. No. 3,862,861, but their cycle-life isstill restricted.

The search for alternatives to the lead-acid battery has been ongoing.As far back as 1934, Drumm disclosed the nickel oxide-zinc battery andthe silver oxide-zinc battery (U.S. Pat. No. 1,955,115). Both of thesebatteries employ zinc as the negative electrode and caustic potash asthe electrolyte. Nickel oxide or silver oxide serves as the positiveelectrode. These batteries have improved energy densities and for manyuses are a good compromise.

The ideal storage battery would combine the best features of existingbatteries with none of the drawbacks. The need for such a battery isapparent for backup systems and in mobile applications. Therefore, it isan object of the present invention to provide an improved storagebattery, one that is both economical and highly efficient. These andother objects, features, and advantages of the invention will berecognized from the following description and accompanying figure.

SUMMARY OF THE DISCLOSURE

A storage battery is fabricated from a positive electrode of lead and anegative electrode of zinc. During charging some lead is converted tolead dioxide. Upon discharge lead dioxide is reduced to lead oxide andzinc is oxidized to zinc oxide. These reactions are reversible such thatthe battery fulfills both functions of a secondary battery: supplyingelectricity on demand and storing or accumulating surplus electricity.

The electrolyte of a cell is alkaline. Aqueous solutions of basesprovide the alkalinity. These bases include ammonia and the hydroxidesof the alkali metals, namely, lithium, sodium, potassium and cesium. Inaddition, tetramethyl ammonium hydroxide may be employed.

Certain additives have been found to be effective buffers in theelectrolyte. These additives include carbonates, borates, silicates andphosphates. They may be introduced by the corresponding acids or theirrespective salts.

The electrodes of a practical embodiment of the invention may beconfigured as sheets, fibers, or particles thereby to maximize electrodesurface area. Interspersed particles of a carbonaceous material may beused to improve the electrical conductivity. A gelling agent may beadded to immobilize the electrolyte. As required, a separator may beemployed between the positive and negative electrodes to prevent a shortcircuit.

Written Description

The chemistry of the lead-zinc battery is important in order to gain anunderstanding of its operation. A positive electrode comprises leaddioxide which is reduced to lead oxide during discharge. The negativeelectrode comprises zinc which is oxidized to zinc oxide when the cellis discharged. The electrolyte is alkaline such that the solutioncontains an excess of hydroxyl ions. The electrode reactions duringdischarge can be represented by the following equations:

Positive electrode:PbO₂+H₂O+2e⁻→PbO+2OH⁻  (1)

Negative electrode:Zn+2OH⁻→ZnO+H₂O+2e⁻  (2)In the above reaction, zinc hydroxide may be an intermediate in theformation of zinc oxide. When these equations are combined, the reactionfor the cell is:PbO₂+Zn→PbO+ZnO  (3)In the overall reaction, there is no change in the average compositionof the electrolyte during discharge although there may be concentrationgradients.

During recharging of the cell, the reactions are reversed. Thus, leadoxide is oxidized to lead dioxide and zinc oxide is reduced to zincmetal. The emf necessary for charging is supplied by an external powersource. The discharge-recharge cycle can be repeated endlessly, thusfulfilling the function of a storage battery.

A particularly difficult challenge in designing new batteries isidentifying electrode materials that will undergo electrochemicalreactions and still withstand corrosion by the electrolyte. Althoughtheory is helpful in this respect, empirical data are required to provethe effectiveness of materials—both for the electrodes and theelectrolyte. One measure of the relative performance of a cell is itsopen-circuit voltage.

The use of lead in an alkaline cell may seem questionable because leadin the +2 oxidation state commonly forms plumbous salts containing thepositive divalent ion Pb⁺⁺. However, by the action of hydroxides onplumbous compounds it is possible to form the negative ion HPbO₂ ⁻ whichis soluble in aqueous solutions. Accordingly Pb(OH)₂ is regarded as anamphoteric hydroxide. In a similar manner, concentrated solutions ofalkali hydroxides act upon the dioxide PbO₂ to form plumbate ions,PbO4⁻⁴ and PbO₃ ⁻², which are likewise soluble.

In view of these considerations, one goal of the research on new cellswas to control the concentration hydroxides in the electrolyte. Thisresult was made possible by employing solutions of sodium carbonatewhich react as follows:Na₂CO₃+H₂O

NaOH+NaHCO₃  (4)From this equation it is seen that such solutions are strongly alkaline.The carbonic acid set free on hydrolysis does not escape when the baseis strong but forms the bicarbonate. However, hydrolysis can be reducedby increasing the concentration of the sodium carbonate, thus permittinga degree of control over the formation of hydroxide.

In place of carbonates, borates can be employed to similar advantage.Boric acid is a weak acid, much more mild than carbonic acid. Thus, itssalts tend to hydrolyze in solution. The following equation shows thereaction of potassium meta borate in solution to form potassiumhydroxide and potassium tetra borate.2K₂B₂O₄+H₂O

2KOH+K₂B₄O₇  (5)Again the hydroxyl concentration can be controlled by adjusting theconcentration of the potassium borate.

Carbonates and borates are effective not only in controlling thealkalinity of the electrolyte, but they also form insoluble salts withlead and zinc. In this manner the corrosion of such electrodes can beminimized. Not only are carbonates and borates helpful in this regard,but other salts are likewise effective. Both silicates and phosphatesform insoluble salts with lead and zinc.

Alkalinity can be provided by compounds of the alkali metals includinglithium, sodium, potassium, and cesium. Lithium has certain limitationsinasmuch as its carbonate and phosphate are almost insoluble in water.Cesium provides a very strong base but the cost of this material limitsits potential applications. While ammonium hydroxide is basic insolution, its volatility restricts its use. Finally, tetramethylammonium hydroxide is known to be strongly alkaline, approaching that ofsodium hydroxide and potassium hydroxide.

The present invention covers the use of aqueous solutions for theelectrolyte. These solutions have the advantage of superior electricalconductivities. Although use of organic solvents including alcohols andglycols is feasible, their performance is inferior.

The configuration of a lead-zinc cell is not restricted. For purposes oftesting various combinations of electrodes and electrolytes, a simplecell was assembled from a glass jar and strips of metals separated, asneed be, by a polypropylene sheet. A workable battery, however, wouldnecessarily be designed with the maximum surface areas for theelectrodes and minimum volume of electrolyte. Such geometric designs asparallel plates, either flat or spirally wound, are appropriate.Alternatively, particles of lead and zinc either alone or interspersedwith graphite may be employed. In this manner, the capacity of the cellcan be increased and its internal resistance minimized.

To gain a greater appreciation of the present invention, FIG. 1illustrates its distinctive features. The cut-away perspective shows alead-zinc battery comprising a single cell with its electrodes arrangedas flat parallel plates. The lead positive electrodes 1 and the zincnegative electrodes 2 are kept apart by separators 3. These parts areimmersed in the alkaline electrolyte 4, which is contained in casing 5.This sectional view also shows the electrical leads attached to theelectrodes. An advantage of this design is that by placing the positiveand negative electrodes in close proximity to each other the quantity ofelectrolyte is reduced.

Applications of a secondary battery as provided by the present inventionare almost limitless. The largest application is in vehicles includingautomobiles powered by new hybrid motors. Other uses include portableelectronic devices such as cell phones and laptop computers.

EXAMPLES

-   -   1. The electrolyte in this example contained sodium carbonate.        The sodium carbonate solution was prepared by heating 96.8 g. of        baking soda (sodium bicarbonate) to 500° F. and dissolving the        product in 185 ml. of water. The positive electrode was formed        from a 1½ in. wide strip of lead obtained from a plumbing supply        business. The negative electrode was a 1½ in. wide strip of zinc        which had been removed from a flashlight battery of the        Leclanche type. The cell compromised a glass jar about 2¾ in.        diameter by 2½ in. high. After charging the cell for 23 minutes        at 2.50 v., an open circuit voltage of 2.32 v. was observed for        the cell. The cell was repeatedly discharged and charged during        the course of the run. At the end of the experiment, the        electrodes were in perfect condition and the electrolyte was        water-white.    -   2. In this run the electrolyte was a solution of potassium        borate. The same electrodes and container were used as in        example 1. To prepare the electrolyte 40.0 g. of potassium        hydroxide and 44.5 g. of boric acid were dissolved in 175 ml. of        water. After charging the cell for 39 minutes at 2.50 v., the        open circuit voltage of the cell was 2.1 v. No corrosion was        apparent on either electrode at the end of the run.    -   3. Tetramethyl ammonium borate was employed for the electrolyte.        The same electrodes were used as in the prior runs, but a        smaller glass jar 2 in. diameter by 3¾ in. high was substituted.        The electrolyte was prepared by dissolving 50.0 g. of        tetramethyl ammonium hydroxide pentahydrate and 19.9 g. of boric        acid in 100 ml. water. An open-circuit voltage of 2.18 v. was        obtained after charging the cell at 2.50 v. for 32 minutes. Both        electrodes were in excellent condition at the end of the run.    -   4. In this case the electrolyte comprised sodium hydroxide,        sodium carbonate, sodium silicate and sodium phosphate. The        electrodes and cell were identical to the ones in the previous        run. The electrolyte was prepared by adding 50.1 g. of a        household dishwasher detergent to 160 ml. water. The detergent        contained sodium carbonate, sodium silicate, enzymes, and 7.4%        phosphorous in the form of phosphates. Activated carbon was        added to the solution before filtering it through four coffee        filter papers. 125 ml. of filtrate was obtained. Next 5.0 g. of        sodium hydroxide was added to the filtrate to produce the        electrolyte. After extensive charging at 2.5 v., an open-circuit        voltage of 2.1 v. was realized. The cell capacity was equal to        or better than the results for any of the other runs. The        electrodes were in perfect condition at the end of the        experiment.

1. A storage battery comprising: (a) a positive electrode of lead; (b) anegative electrode of zinc; and (c) an alkaline electrolyte.
 2. Astorage battery of claim 1 in which the alkaline electrolyte is anaqueous solution of a hydroxide of an alkali metal selected from thegroup lithium, sodium, potassium, and cesium.
 3. A storage battery ofclaim 1 in which the alkaline electrolyte is an aqueous solution oftetramethyl ammonium hydroxide.
 4. A storage battery of claim 1 in whichthe alkaline electrolyte contains a buffering agent selected from thegroup carbonates, borates, silicates, and phosphates.